Integrating Microfluidic and Biosensors: A Mini Review

In recent years, the field of analytical research has witnessed a significant transformation driven by the emergence of integrated microfluidic sensors. This ground-breaking technology has been extensively studied, resulting in the resolution of diverse challenges and a revolutionary impact on experiments, particularly in the biomedical domain. By combining the biosensors with microfluidics, there is a tremendous potential to enhance measurement accuracy and expand the capacity of specimens utilized in biomedical applications and experiments. The integration of biosensors with microfluidics enables effective sample separation, precise control over chemical reactions, and the measurement of various critical parameters. Furthermore, the primary objective of this research is to identify gaps in the existing literature concerning integrated microfluidic sensors. This pursuit involves employing comprehensive bibliometric analysis and conducting a systematic literature review of Scopus-indexed publications that are relevant to the field of integrated microfluidic sensors. PRISMA method was being used to filter the documents that are gathered from Scopus database. The outcomes of this study underscore the pressing need for further research in leveraging electrochemical sensors for specimen analysis by integrating them with the advanced technique of microfluidics. The paper emphasizes the significance of continuous research and development efforts in the realm of integrated microfluidic sensors to fully exploit the potential of electrochemical sensors and enhance the overall research process.


Introduction
The advancement of technology in recent years has rapidly progressed in various fields, including healthcare, industry, sensorics, and chemical analysis.This technological development has led to the emergence of various fields, such as microfluidics, which have the potential to transform and enhance various other areas such as healthcare, the environment management, and chemical analysis within living organisms for disease detection [1].Microfluidic systems offer several advantages, allowing for control and manipulation of minute fluid volumes on a microscale for detection.Once these fluid volumes are detected, they can be efficiently and effectively analyzed [2].Currently, there are applications that serve as the primary domain for integrating sensors with microfluidic platforms, forming the foundational basis of integrated microfluidics sensors.
Integrated microfluidic sensors combine microfluidic techniques with sensor technology capabilities, enabling integrated analysis, manipulation, and compact sensor capabilities [3].This offers several advantages compared to traditional detection methods.Additionally, integrated microfluidic sensors have the ability to perform highly sensitive real-time measurements using minimal specimens of molecules.The numerous advantages of integrated microfluidic sensors make them an ideal application for high-speed analysis with high accuracy [4].Consequently, their implementation has been utilized in various sectors, particularly in biomedical diagnostics, environmental monitoring, and process control in various industries.
This review paper aims to provide a comprehensive analysis of the advancements in the field of integrated microfluidic sensors.The paper will identify and review various papers, analyses, and theoretical foundations proposed by experts in the field of integrated microfluidic sensors.The review paper will not only explore the discoveries that have been made but will also identify the existing research gaps up to the present.The research related to integrated microfluidics with electrochemical sensors is conducted to identify research gaps.Various studies and papers proposed by experts will be used as the basis for systematic literature review and bibliometric analysis, utilizing a database sourced from Scopus.
The identification of various papers and research conducted using the Scopus database will be processed in the Publish or Perish software.The processing is done with the aim of obtaining papers related to the topic of integrated microfluidics, which will be used as data for systematic literature review and bibliometric analysis.This research aims to analyze sensor techniques that are closely related to integrated microfluidics but have been less explored.The findings will be visualized through bibliometric analysis, providing insights that can significantly contribute to the development and impact of integrated microfluidic sensor studies.

Correlation between Integrated Microfluidic and Sensors
Amidst a rapidly progressing era, numerous disciplines have emerged to address the requirements of academic research, particularly in the field of clinical diagnostics [5].Among these disciplines, microfluidics has gained notable recognition in this swiftly evolving period.Microfluidic devices, characterized by their small-scale or miniature nature, offer a multitude of versatile applications encompassing sample preparation, liquid separation, and the identification of substance constituents within a liquid medium [6].The development of sensitive and reliable sensors stands as a significant challenge in the advancement of microfluidics.Sensors serve as crucial tools in quantifying the physical or chemical properties of substances.The integration of microfluidics with sensors has propelled technological advancements and yielded innovative solutions applicable across a wide spectrum of research needs.The amalgamation of microfluidic and sensor integration facilitates precise chemical detection and enhanced efficacy in clinical diagnostics [7].Biosensor process flow illustrated through schematic diagram can be seen on Figure 1.
Figure 1 Biosensor process flow illustrated through schematic diagram [8] The role of sensors in integration with microfluidics is found in detection, while the role of microfluidics in integration with sensors is sample preparation.A study where integrated microfluidics is combined with EV detection to fractionate plasma, quantify, and analyze proteins directly from blood samples at high speed.The aim is to detect and monitor ovarian cancer [9].The detection and monitoring process of ovarian cancer using integrated microfluidic device can be seen in Figure 2. Furthermore, the integration of microfluidics and sensors can be used for continuous monitoring of samples within the microfluidic system.Detection is performed using various biosensors assisted by microfluidics in separating different substances.The monitoring process takes place in situ, allowing long-term monitoring of drug-induced organ toxicity [10].The monitoring process for drug-induced organ toxicity can be seen in Figure 3.The integration of microfluidics and sensor technology can be accomplished through multiple methods.One frequently utilized approach is monolithic integration, which involves fabricating integrated microfluidics and sensor technology on a single substrate during the fabrication stage.This approach presents several advantages, including excellent product performance, costeffectiveness, and the ability to easily miniaturize the system [11].Another viable approach is modular integration, which provides increased flexibility.Nonetheless, this method also has certain limitations, including heightened implementation complexity and higher costs [12].Therefore, each approach has its own advantages and disadvantages, and the effectiveness of each approach depends on its specific goals.The integration of microfluidics and sensors holds immense potential to revolutionize a wide range of industries.The combination of these two elements enables the development of new and innovative analytical technologies.For instance, integrated microfluidics and electrochemical sensors can be utilized to detect various small molecules, such as glucose, DNA, and bacteria [13][14].

Electrochemical Sensors
The rapid advancement of technology has led to the development of devices that can convert chemical information into electrical signals.These devices are known as electrochemical sensors [15].Electrochemical sensors have been used in recent decades to detect a wide range of substances, from ions to molecules to cells.They offer several advantages over traditional methods, including high sensitivity, efficient and effective analysis, and cost-effectiveness [16].As a result, they have attracted attention for their potential applications in disease diagnosis.
Electrochemical sensors have the capability to diagnose diseases such as cancer, diabetes, and infectious diseases [17][18][19].This diagnostic process involves measuring the electrical potential that is generated when chemical reactions occur at the surface of the electrode.The use of electrochemical sensors is particularly noteworthy in the detection of cancer biomarkers in specimens such as blood and urine [17].This enables early-stage cancer detection, facilitating timely intervention.Moreover, in the context of conditions such as diabetes, electrochemical sensors are utilized to identify the levels of glucose in individuals.This aids in the continuous monitoring of blood glucose, thus helping to prevent potential complications [18].
Generally, there are two types of electrochemical sensors: potentiometric sensors and amperometric sensors.Potentiometric sensors possess biodegradable properties and are more environmentally friendly compared to other electrochemical sensors.This is due to the utilization of paper as a substrate in the fabrication of potentiometric sensors [20] .Potentiometric sensors of this type can provide information regarding ion activity without being dependent on the sample volume [21].These sensors are based on polymeric membrane ion-selective electrodes (ISE) and ionselective field-effect transistors (ISFET), enabling the measurement of potential changes at an electrode [22].
Another type of electrochemical sensor is the amperometric sensor, which operates by applying voltages where the electrode reactions are determined by the rate of gas transport [23].In recent years, innovations have been made in the development of semi planar-shaped amperometric ethylene sensors.These new findings offer various advantages, such as increased sensitivity to solid polymer electrolytes (SPE) and low cost [24].Furthermore, Amperometric sensors categorize most blood glucose meters [25].

Methods
A research study employing bibliometric analysis and systematic literature review was conducted using the Scopus database in May 2023.Data collection was performed using the Scopus database, focusing on the fields of sensors and integrated microfluidics.Scopus was chosen as the database due to its various advantages, including a wide range of high-indexed scientific articles and journals.The process of identifying relevant scientific literature for the research topic, conducting bibliometric analysis, and performing a systematic literature review followed the PRISMA method.The keywords used for searching relevant scientific articles in the Scopus database were "sensor," "integrated," and "microfluidic."The keyword search was conducted in the title, abstract, and keyword fields of numerous articles.The search algorithm employed a Boolean approach, resulting in the search query "TITLE-ABS-KEY (sensor AND integrated AND microfluidic)." A total of 3,178 records were obtained from the Scopus database.The next step involved screening the data using the PRISMA method.The screening process was conducted according to several criteria, including articles written in English, open access documents, and publications published no earlier than 2018.Subsequently, the Publish or Perish software was used to screen articles with high impact.Then, the high impacts article will be utilized to conduct bibliometric analysis and systematic literature review.In this context, high impact refers to research that has made significant contributions to the fields of integrated microfluidics and sensors.The screening process using the PRISMA method is visualized in Figure 4.

Systematic Literature Review
A systematic literature review was conducted using 16 selected papers obtained from the Scopus database, focusing on the integration of biosensors and microfluidics.Figure 4 illustrates the screening process employed by the researchers to obtain the 16 selected papers for the systematic literature review.These selected papers provide valuable information and outcomes to researchers in the field of microfluidic and biosensor integration, offering potential solutions to future challenges and problems.The summary obtained through these selected papers suggests that the integration of microfluidics and biosensors holds promise for the detection of various diseases through biomarkers and relevant parameters.Table 1 presents a summary of the systematic review conducted in this study.The ability of the electrochemical microfluidic biosensor lies in its capacity to detect miR-19b in serum samples of children suffering from brain cancer.Verification can be performed through the utilization of the polymerase chain reaction method, supported by electrochemical CRISPR, which offers cost-effectiveness, easy scalability, and target amplificationfree advantages.[5] Integration of On-Chip and avalanche silicon light-emitting Solution dissolved biomolecules, gasses or volatile organic compounds Micro optical sensors have the ability to be combined with adjacent driving and optical processing circuits, offering benefits that include the reduction in size and the ability to be produced in large quantities using standard Silicon.This integration enables the development of optical sensors that are compatible with CMOS technology.[26] Wearable Colometric Sensor Based on Microfluidic Chip Technology

Sweat Glucose
This research aims to develop a wearable device supported by microfluidic technology for detecting glucose levels in sweat samples using colorimetric analysis techniques.The obtained results from the device successfully detect variations in glucose levels in sweat between fasting and sugar postmeal subjects. [ A Paper-Based Immunosensor With Integrated Electrochemical Capabilities, Modified Using Nanocomposites of Multi-Walled Carbon Nanotubes.

17β-estradiol (17β-E2)
Experiment for detecting 17-β E2 is developed using an electrochemical immunosensor, which offers advantages such as easy mass production at a low cost, accurate sensitivity, and ease of testing.Additionally, the sensor can detect and transmit data via Bluetooth to a smartphone; however, it has a lower level of accuracy. [28] Nanotextured Glucose Sweat Sensors Utilizing a Porous Enzymatic Membrane.

Glucose
This research has successfully developed nanostructured glucose sensors that utilize nanoporous membranes and dendritic nanostructures to enhance enzyme immobilization, increase surface area, and prevent delamination.These sensors can be utilized at the point of care as they can be integrated into a printable and wearable patch sensor with wireless transmission capabilities. [29] Three-dimensional paper-based microfluidic electrochemical integrated devices (3D-PMED)

Glucose on sweat
This research describes the development of a three-dimensional paperbased microfluidic electrochemical integrated device (3D-PMED) for constructing wearable sweat monitoring devices.The device effectively controls sweat flow and prevents sweat accumulation.The 3D-PMED has also been successfully integrated with a glucose sensor, enabling the monitoring of glucose levels within sweat. [14]

Incorporating
Inkjet-Printed Sensors Within Liver-On-A-Chip Technology Oxygen, pH, glucose and lactate The integration of a dissolved oxygen sensor is achieved by creating a thin, flexible, and porous cell culture membrane using biocompatible dielectric ink.Successful measurement of oxygen gradients is performed in liver cell cultures, demonstrating reliable real-time monitoring of mitochondrial respiration.[30] MXene-Graphene Field-Effect Transistor Influenza Virus and SARS-CoV-2 Sensors designed to detect influenza virus and SARS-CoV-2 offer advantages of being low-cost and easy to produce.These sensors also demonstrate a low limit of detection with a wide detection range and fast response time.Their high sensitivity to significant changes makes them suitable for virus detection in environmental settings and for sampling in challenging conditions.[ This research demonstrates a practical laboratory on a chip system that utilizes a soft microfluidic network integrated with flexible electronics.This system enables the detection and monitoring of both physical and mental aspects.The designed principles can be tailored to incorporate various additional functions for analyzing Eccrine sweat.[32] Non-Enzymatic Glucose Sensors Utilizing Laser-Induced Graphene.

Nonenzymatic glucose
The porous Laser-Induced Graphene (LIG) foam retains its porosity and demonstrates good sensitivity by eliminating the need for alkaline samples, enabling glucose measurements.LIG sensors exhibit sensitivity that can be utilized for stem cells and sweat analysis.Integration with a microfluidic channel allows for glucose monitoring to be conducted.[33] Capacitive Aptasensor Coupled with Microfluidic Enrichment

SARS-CoV-2 Nucleocapsid Protein
This research demonstrates a low-cost aptasensor capable of rapidly detecting SARS-CoV-2 infection directly.The sensor utilizes specific aptamers to detect the N-protein of the virus in saliva, making it suitable for the detection of asymptomatic individuals.Overall, this aptasensor offers a solution for widespread detection of SARS-CoV-2.

Glucose on sweat
This research primarily aims to develop a thermoresponsive microfluidic system integrated for detecting glucose in human sweat.The system allows quantitative evaluation of glucose by integrating it with a smartphone.This device is cost-effective and capable of detecting and monitoring glucose levels in sweat, including in the clothing of individuals working in hightemperature environments.[35] Affordable Biosensing Platform Uses Sin Nanophotonics

Antigen and protein in urine
The summary of this research describes an integrated sensor capable of detecting various biomarkers at an affordable cost.The proof of concept results demonstrate the detection of LAM in urine samples and CRP, indicating successful biomarker detection.[ This study presents successful progress in the creation of a sensor system designed to continuously monitor lactate levels in sweat.By employing 1,2-Naphthoquinone as a mediator and integrating a PDMS microfluidic sweat collector, the performance and user-friendliness of the system have been enhanced.The system demonstrates a remarkable ability to detect low concentrations of lactate, exhibits a broad range of detection capabilities, and is compatible with long-term use. [37] Paper-Based Antibody Detection Devices Using Bioluminescent BRET-Switching Sensor Proteins Antibody This research introduces an analytical system that utilizes a ratiometric bioluminescence detector in a single drop of blood.The system consistently exhibits a reliable response across a wide range of sample sizes.Furthermore, the integration of 3D-μPADs with other BRET sensors enables the detection of small-molecule drugs and nucleic acids. [ An Integrated Adipose-Tissue-On-Chip Nanoplasmonic Biosensing Platform For Investigating Obesity-Associated Inflammation

Cytokine
The platform developed in this research enables the measurement of multiple cytokines released by adipose tissue cells in response to inflammation.By combining LSPR barcode with antibodies, both proinflammatory and anti-inflammatory cytokines can be simultaneously detected.The potential of this platform's development can be utilized in obesity prevention and drug detection applications. [39]

Bibliometric Analysis
Bibliometric analysis is utilized for analyzing books, journals, publications, and other scholarly sources.In this study, the researcher conducted a review using VOSviewer and Publish or Perish to identify suitable journals to support the success of this research.A total of 34 publications were found after filtering them using the Publish or Perish application.Subsequently, these 33 publications were processed using the VOSviewer application.The obtained results revealed several keywords that support the research on the integration between microfluidics and electrochemical analysis, such as electrochemical detection, disease, microfluidics, and others.Figure 5 illustrates the visualization of data and keywords related to integrated microfluidics.The aforementioned data visualization was generated by analyzing 304 journals using the PRISMA methodology.The VosViewer data visualization illustrates the presence of four distinct clusters: red, blue, yellow, and green.The red cluster primarily pertains to the utilization of microfluidic sensors for monitoring biological and chemical substances within the human body.These sensors enable the monitoring of glucose levels in blood and sweat, as well as the detection of COVID-19 biomarkers within the body.The blue cluster represents the implementation of microfluidic sensors (CMOs) for the detection of antibodies, proteins, and other particles in disease diagnosis.On the other hand, the yellow cluster focuses on microfluidic sensors integrated with chips designed for culturing cells and organs.Lastly, the red cluster centers around electrochemical sensors capable of detecting bacteria that are sensitive to electrical currents.
The visualization results reveal a gap in the integration of microfluidics with the use of sensors for disease analysis, cell culture, and organ on chip.However, the integration of sensor such as electrochemical sensor with microfluidics has the potential to greatly improve the efficiency of disease detection, cell culture, and organ on chip.One promising parameter that can be utilized for disease detection through the integration of microfluidics and electrochemical sensors is miRNAs [5].This advancement could significantly contribute to the development of clinical diagnosis in the healthcare industry in the future.The overlay visualization results in Figure 6 reveal that darker colors (purple) indicate research conducted in 2020, while brighter colors (yellow) represent research conducted in 2021.The most recent study focused on integrated microfluidics and electrochemical analysis was conducted in 2020.In contrast, the latest research on sars cov, cell culture, and organ on chip took place in 2021.This finding highlights the potential benefits of integrating microfluidics and electrochemical sensor, which can significantly impact the healthcare industry.Electrochemical sensors are widely recognized as effective tools for detecting biomarkers associated with diseases, cell culture, and organ on chip [10][16][30] [31].
The density visualization results in Figure 7 indicate that as the visualization becomes denser, the color becomes brighter (yellow), which signifies that the topic is frequently discussed by researchers.On the other hand, visualizations that are less dense and sparse have darker colors.For example, in the bibliometric analysis using VOSViewer, the topic of microchannel is represented by the yellow color with an occurrence of 31, indicating its high frequency of discussion among researchers.In contrast, the topic of sars cov is represented by a darker color with an occurrence of 9, suggesting that it is less frequently discussed.
The analysis conducted has identified a gap between microfluidic sensor with sars cov, cell culture, and organ on chip when combined with biosensors such as electrochemical sensor.A recommendation for future research is to explore the relationship between microfluidic and biosensors.It is well-known that biosensors integrated with microfluidics can be used for sample preparation and disease detection [13].Considering the rapid advancements in technology and the positive impact that can be achieved through the integration of biosensors and microfluidics, further research on the integration of microfluidics and biosensors is crucial for the healthcare industry.

Conclusion
This research utilizes two analysis techniques: systematic literature review and bibliometric analysis to analyze the topic of microfluidic and biosensor integration.The data for this study were obtained from the Scopus database using keywords such as sensor, integrated, and microfluidic.Several inclusion and exclusion criteria were applied to screen the data, resulting in 303 papers used for bibliometric analysis and 16 papers used for systematic literature review.The systematic literature review technique was employed to provide a summary understanding of the published papers.Meanwhile, bibliometric analysis was used to identify research gaps based on the topics that have been previously investigated.The research gap identified in the bibliometric analysis is the integration of microfluidics and biosensors for the early detection of various diseases as an initial diagnosis.
Through the analysis conducted on the integration of biosensors and microfluidics, various additional findings have been obtained.Regarding the parameters that can be used for the integration of microfluidics and biosensors in detecting various diseases, several parameters have been identified.Commonly used parameters include DNA, RNA, viruses, proteins, sweat, antibodies, and others.Based on the bibliometric analysis, a research gap was identified in the detection of diseases, cell culture, and organ on chip using the integration of microfluidics and biosensors, such as electrochemical analysis with antibodies as parameters.For future steps, researchers are recommended to encourage exploration and research on disease detection, cell culture, and organ on chip using the integration of microfluidics, biosensors, and biomarkers (DNA, viruses, and protein) as parameters.Based on the analysis conducted, research on the integration of microfluidics and biosensors can contribute to the development of early diagnosis in the healthcare industry.

Figure 2 Figure 3
Figure 2 Schematic Diagram of Plasma Separation Using Integrated Microfluidic and Biosensor [9].Reprinted (adapted) with permission from Zhou, S, et al.Copyright © 2019 American Chemical Society

Figure 4
Figure 4 Process of Conducting Scientific Literature Review with PRISMA Method

Figure 5
Figure 5 Network Visualization of Integrated Microfluidics Using Bibliometric Analysis

Figure 6 Figure 7
Figure 6 Overlay Visualization of Integrated Microfluidics Using Bibliometric Analysis

Table 1
Systematic Literature Review